In many flow control applications there is a need for structures which can vary the fluid-flow rate of flowing fluids without the production of noise and vibration. The term "throttling" is generally applied to the function of altering or adjusting fluid flow throughout a range of flow rates. The various structures by which the function is performed are generally called "throttling valves" to distinguish them from structures whose function is to open and close a flow path as a step function. To the extent that on-off valves are not opened and closed instantaneously, so that throttling noise and vaibration may be produced therein at the time of opening or closure, the invention described herein is applicable to such valves, as well, and they are included in the term throttling valve.
A typical control valve for handling the flowing of high pressure fluids employs a structure in which the cross-sectional area of the flow path is altered. This type of structure generally produces substantial noise and vibration and is quite subject and in damage from cavitation. However, the structures employed in this arrangement are, as a class, least expensive and most conveniently employed. Of particular interest herein is a structure for quieting of and prevention of damage to spool valves. In general the noise, vibration and cavitation generated in orificial valves is an incident to the Venturi effect which attends movement of the fluid through the orificial opening. The energy difference from throttling results in turbelence following the orifice where it is transformed into increased internal temperature of fluid and into acoustic energy in the form of noise transmitted through the fluid and in8c vibration damage to the surrounding structure. In extreme cases, the turbulence results in localized pressure reductions downstream from the orficial restriction sufficient to form vapor spaces or pockets. The vapor in these spaces is returned to liquid as the vapor bubble is imploded by the pressure of the medium surrounding the bubble. This phenomenon is called cavitation and results in noise and occasional damage to adjacent surfaces of the valve structure. It will be appreciated that there are many applications for which it is desired to substantially reduce both the noise and the valve damage in operation of spool valves. It is also suspected that some damage may result from a molecular shearing phenomenon wherein, because of forcing fluids such as hydraulic oil through small areas with very high pressure differentials, electrons are actually separated from oil molecules which may be replaced from the surrounding metal, thus causing another form of erosion.
There have been many structures devised in an attempt to deal with the noise, vibration and damage resulting from operation of valves in high pressure systems. Most of these have involved some form of baffling means which operate in one way or another to divide the flow into finely divided streams. One such arrangement is described in the copending application Ser. No. 470,251 referred to above, in which flow is divided into many fine streams by a series of stacked disks surrounding a spool valve and in which each small stream is caused to flow into a chamber, from thence across an orifice to another chamber, reversing direction through another orifice, etc., and so on radially across the disks. In this arrangement the pressure drops across the disks are essentially those caused by the orifices in series. One problem which has been experienced with this arrangement is that the disks containing the orifices are not configured to receive or discharge fluid, nor are the blank disks. Thus, particularly where a spool valve has very small travel, the thickness of these dead disks creates an irregularity in flow which it is preferable to avoid.